Tritium removal device for lithium loop

ABSTRACT

A tritium removal device for a lithium loop comprises a neutron source ( 1 ) for colliding proton on a lithium flow, thereby generating neutrons, a lithium tank ( 11 ) for letting the lithium passing through this neutron source ( 1 ) to flow into through a flow passage ( 9 ), thereby for temporality accumulating it therein, and a lithium pump ( 17 ) for circulating and supplying the lithium of this lithium tank ( 11 ) to the neutron source ( 1 ) through a supply-side flow passage ( 9 ′). The lithium tank ( 11 ) and the lithium pump ( 17 ), into which hydrogen gas containing tritium therein can be easily collected, are enclosed within a hermetically sealed container ( 7 ) including an inactive gas therein, so that even if the hydrogen gas including the tritium therein is leaked into the hermetically sealed container ( 7 ), it is removed by a hydrogen isotope removal filter. It is possible to remove the tritium, with safety, without diffusing the tritium from an inside of the lithium loop for forming a target flow into an atmosphere.

TECHNICAL FIELD

The present invention relates to a tritium removal device for removinghydrogen and tritium, being an isotopic element thereof, which aregenerated on a lithium target to be applied as a neutron source, from alithium loop, in which lithium flows=circularly, for example, in a boronneutron capture therapy (BNCT), which is applied for the purpose of acancer medical treatment.

BACKGROUND OF THE INVENTION

The boron neutron capture therapy (BNCT) is a medical treatment achievedfor the purpose, i.e., selectively killing cancer cells, by “a” rays(helium nucleus) and secondary particle radiation of lithium nucleus(⁷Li), which are generated through neutron capture nuclear reaction from¹⁰B of a boron isotope (¹⁰B) compound having a large neutron capturecross section, which is introduced into the cancer cells in advance, byirradiating thermal neutrons having relatively low energy, such as,equal to 0.5 eV or less than that, upon the cancer cells. Because arange of the “α” rays and so on is very short, only the cells taking ¹⁰Btherein are destroyed, and therefore this attracts attentions as amedical treatment against the cancer, producing less unfavorable sideeffects.

In an initial boron neutron capture therapy (BNCT), there are used theneutrons from a nuclear reactor through deceleration thereof; however,at present, there are two types, i.e., applying the neutrons, which aregenerated by irradiation of protons accelerated through an acceleratorupon solid Be cooled with water,=and also applying the neutrons, whichare generated by irradiation of protons accelerated through anaccelerator upon liquid lithium. Herein, explanation will be given onthe latter, i.e., the method of applying the neutrons, which aregenerated by irradiating the protons upon the liquid lithium.

The method applying the liquid lithium therein as a target material hasadvantages that it enables to remove heats, continuously, by circulatingthe lithium, and that less fission poison is generated through a nuclearreaction by selecting an irradiating energy equal to 8 MeV or less thanthat. In the present invention, ⁷Li is changed to ⁷Be upon irradiation,but ⁷Be turns back to ⁷Li, again, after a half-life period, fifty-three(53) days; i.e., it can be used continuously. Although being aradioactive material, ⁷Be is enclosed within a lithium loop under thecondition of being dissolved into the lithium, and it turns back tonon-radioactive ⁷Li accompanying with an elapse of time. Further,determining the irradiating energy of the protons, to be equal to 2.0MeV or less than that, brings about an area or region for generatingneutrons, upon which no deceleration is necessary, and therefore, it ispreferable to determine the irradiating energy to this, i.e., equal to2.0 MeV or less than that, and also equal to 1.881 MeV threshold valueenergy or greater than that, at which the neutrons are generated. This⁷Be generating in the nuclear reaction depending on the irradiatingenergy is one over several tens comparing to 2.5 MeV, as a source ofgenerating neutrons, for use in the boron neutron capture therapy(BNCT), and it is the best to apply the protons of this energy band.

In this boron neutron capture therapy (BNCT) is applied a thermalneutron, equal to 0.5 eV or less than that, which is generated uponcollision of the protons on a lithium target. In this instance, as anexample of the reaction in the vicinity of the threshold value, at whichno moderator is necessary to the neutron, determining the energy of theproton at 2 MeV, and current thereof to 20 mA, results into that a largethermal energy of 40 kW is given to the lithium. In spite of inputtingthis large heat therein, it is better for the lithium not to evaporate,and for the purpose of enabling a continuous operation thereof, thereare the following necessities; i.e., that the lithium can always passthrough a target portion of the neutron source, at high-speed and withstable thickness, so as to suppress the lithium from an increase oftemperature thereof, and that the lithium as the target is alwayscirculated through a lithium loop, which is built up to have anequipment for removing the heat from lithium on a way of the lithiumloop, i.e., a closed loop of lithium. The lithium of the target portionof the neutron source is formed, being as thin as possible, for notdisturbing loci of the neutrons generating into a direction ofirradiation of the protons, and also for suppressing attenuation of theneutrons, and further for forming a stable lithium target stream madefrom a thin laminar flow, having such a thickness of 0.5 mm,approximately, being equal to 0.25 mm or greater than that in the depththereof, into which the protons irradiated, being so-called a “braggpeak”, are absorbed, abruptly, and then onto this is hit a ray ofprotons, thereby generating the neutrons therefrom.

In such proton source as mentioned above, collision of the protons uponthe flow of lithium target generates the neutrons to be applied for thepurpose of the medical treatment. Also, at that time, the proton itselfturns back to hydrogen by taking an electron of lithium in the peripherythereof into, and a part of this hydrogen is dissolved into the lithium,but much thereof reacts on the lithium to become lithium hydride. Ifassuming that the current of irradiating protons is 20 mA, an amount ofhydrogen is only 6.53 g, if all of the protons turn back to the hydrogenand if irradiating thereon, continuously, for one year, and if assumingthat there is 25 Kg of lithium, for example, and that an entire thereofcome to be the lithium hydride, then an amount of the lithium hydrideobtained is 52.24 g; i.e., this reaches only 0.21% of the amount oflithium, and in the condition of being dissolved into the lithium in theform of lithium hydride. On the other hand, the neutrons generating inthe lithium upon irradiation of the protons are taken into ⁶Li, i.e. ,the lithium isotope included in the lithium at 7.4%; therefore neutroncapture ⁶Li are generating tritium, i.e., hydrogen isotope, a little bitthereof, through the nuclear reaction. An amount of tritium generatingupon continuous irradiation of protons for one year is further small,only 2.44 μg, approximately. Nevertheless, the tritium is relativelylong in the half-life period thereof, such as, 12.3 years; therefore, itis difficult to discharge this tritium as it is, but rather necessary toremove the tritium from, thereby not to be discharged into theatmosphere, but to be stored, or the tritium must be attenuated, ifbeing discharged, in a concentration thereof, to be equal to or lowerthan a reference value determined according to a law related therewith.A part of this tritium also dissolves into the lithium, similarly, butalmost thereof reacts on the lithium, thereby bringing about lithiumtritiated.

In this manner, because the tritium is very little in the quantitythereof, and is further small quantity, judging from the amount ofhydrogen reacting upon all of the lithium when there is 25 Kg of thelithium, it is in the condition of being dissolved in the lithium in theform of the lithium tritiated. Also, those, i.e., the lithium hydratedafter reacting upon the lithium and also the lithium tritiated, arestable, in the high temperature thereof; i.e., not decomposed up to 686°C., and therefore is very small in the amount thereof, which isdischarged as a gas in a vacuum discharge system or an argon cover gassystem.

However, for the tritium generating, since there are cases of beingdischarged from the lithium, into the vacuum discharge system or theargon cover gas system, even with a very small possibility thereof, insuch boron neutron capture therapy (BNCT) as mentioned above, it isnecessary to remove this radioactive material, i.e., the tritium, fromamong the lithium loop, not to be discharged into the atmosphere. Also,almost all of the hydrogen and the hydrogen isotope, generating in aflow of lithium during the time of irradiation, come to be a lithiumcompound, being hardly decomposable; however, since a very littlethereof, being dissolved under the condition of hydrogen atom, has apossibility of being gasified in such a place as being low in pressurethereof, etc., for example, that the gas, generating in a flow portionwithin vacuum, from the target portion up to a quench surface where thepressure is low among flows of the lithium and/or in a quench tankportion, forms bubbles and thereby rises up, or that it generates in avacuum portion of an inlet portion of a pump. The gas generating causesto generate cavitations in the inlet portion of the pump, or changeflows before and after a nozzle, or bring the flow of lithium to beunstable. Accordingly, also for circulating the flow of lithium instable, there is a necessity of removing the gas of that hydrogen andhydrogen isotope.

Conventionally, as the technology for removing the tritium generatingfrom a nuclear power facility, etc., there are already proposed severalones, as described in the following Patent Documents 1 to 4, forexample. However, in the conventional arts, no proposal is made of ameans for removing the tritium among from the lithium loop, for suchneutron source as mentioned above. For this reason, in order topropagate such boron neutron capture therapy (BNCT) as mentioned above,widely, in medical facilities, it is desired to develop a lithium targetsystem equipped with a tritium removal device, for enabling to form astable lithium target flow, without diffusing the tritium into theatmosphere, and further to remove the tritium from among the lithiumloop, with safety. Also, it is expected to be put in practical use as atarget system for use in the neutron source of an accelerator drivingtype, other than the boron neutron capture therapy (BNCT).

In general, the tritium (T) cannot exist in the form of a tritiummolecule where the hydrogen exists, but almost thereof exists under thecondition of HT, combining with the hydrogen, and it is very hard toseparate or divide those from each other, and the characteristicsthereof are almost same to; therefore, the tritium must be processedtogether with the hydrogen when trying to remove the tritium from thehydrogen. Among hydrogen isotopes, general hydrogen is called, “lighthydrogen”, distinguishing from deuterium and tritium; however herein,the light hydrogen is called only by “hydrogen”. Also, the gassesincluding the hydrogen and the tritium therein are called, “hydrogenisotope gasses”, and in this device, so as to emphasize or distinguishthe tritium, remarkably, the radioactive material, but always includingthe light hydrogen therein, it is dare to call, not “hydrogen isotopegas”, but “tritium”, in case of the purpose of removal of the tritium.

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Patent Laying-Open No. 2005-127718 (2005);

[Patent Document 2] Japanese Patent Laying-Open No. Hei 06-331791(1994);

[Patent Document 3] Japanese Patent Laying-Open No. Hei 05-341096(1993); and

[Patent Document 4] Japanese Patent Laying-Open No. Hei 02-93399 (1990).

BRIEF SUMMARY OF THE INVENTION Problem(s) to be Dissolved by theInvention

An object of the present invention, accomplished by taking the drawbacksin such tritium removal technologies of the conventional arts asmentioned above into the consideration thereof, is to provide a lithiumtarget system, for enabling to obtain a stable lithium, with preventingthe gas generated in the lithium from generating cavitations in an inletof a pump, or from generating pulsation or fluid vibration in a valve ora nozzle, with applying a hydrogen and hydrogen isotope removal devicefor removing the tritium, with safety, without diffusing the hydrogenand the hydrogen isotope, i.e., the tritium, among from a lithium groupfor forming a lithium target flow.

Means for Dissolving the Problem(s)

The present invention, for accomplishing the object mentioned above,within a lithium loop for flowing lithium circularly, for allowingprotons to reach up to a lithium target portion without colliding on amaterial, comprises a vacuum pump for maintain vacuum withdifferentially discharging the lithium target portion, and on dischargeside of the vacuum pump, a hydrogen isotope removal filter and ahydrogen storage tank, and has a gas system, for accumulating thelithium, once, including hydrogen and tritium therein, generating in thelithium of the lithium target portion, being a neutron source, so as tokeep a necessary NPSH (Net Positive Suction Head) for a pomp inputportion due to a head of lithium liquid, and further for decelerating aflow within the tank, to let the hydrogen and the tritium gas in thelithium to float up and thereby collecting them in an upper portion ofthe tank, for removing gaseous components within the lithium so that nocavitations is generated due to gas, and for sucking the gas includingthe hydrogen and the tritium collected within the tank, therebyadjusting the gas pressure, wherein peripheries of equipment where thetritium gas gathers, including a portion of the pump for turning thelithium back to the neutron source, again, therein, are enclosed in anair-tightly hermetically sealed space of an inactive gas atmosphere,such as, argon, etc., and even in case where the tritium leaks out thelithium loop and an argon gas system thereof, by any change, into thishermetically sealed container, the tritium to be discharged togetherwith a carrier gas from an inside of the hermetically sealed containeris removed by a hydrogen isotope removal filter.

Thus, according to the present invention, there is provided a tritiumremoval device for a lithium loop, comprising: a neutron source 1 forcolliding proton on a lithium flow, thereby generating neutrons; alithium tank 11 for letting the lithium passing through this neutronsource 1 to flow into through a flow passage 9, forming a quench surfaceof a lithium flow in the flow passage 9, and for temporalityaccumulating it therein, into which an exit of the flow passage formingthe quench surface therein is connected, and thereby bringing thelithium tank 11 to be applicable with a gas pressure; and a lithium pump17 for circulating and supplying the lithium of this lithium tank 11 tosaid neutron source 1 through a supply-side flow passage 9′, whereinhydrogen gas containing tritium therein, which is accumulated in a gassystem of the lithium loop is removed by a hydrogen isotope removalfilter, and further said lithium tank 11 and said lithium pump 17 areenclosed within a hermetically sealed container 7, and with thishermetically sealed container 7 is connected a discharge system flowpassage through said hydrogen isotope removal filter.

This tritium removal device, being connected hermetically sealedcontainer 7, enclosed all of gasses, into which the tritium leaks out byany change from the lithium tank 11 and the lithium pump 17 and the gassystems thereof, etc., within the hermetically sealed container 7including the inactive gas therein, once, and the gas included in thishermetically sealed container 7 is discharged into the discharge systemflow passage passing through the hydrogen isotope removal filter. Forthis reason, even in case where the gas leaks out from the lithium tank11 and the lithium pump 17, etc., by any chance, it is possible todischarge that gas, after removing the tritium contained in the gasleaking out.

How much of the tritium is contained in that gas of the gas system, fromwhich the tritium gas is removed, i.e., the partial pressure thereof ismeasured by a quadrupole mass spectrometer Q-mass, accurately, byattaching the quadrupole mass spectrometer Q-mass, being superior inair-tightness, to a connecting tube, after differentially discharging,the gas is diluted with air to be equal to or lower than a referencevalue depending on that partial pressure, and further is dischargedwhile monitoring concentration of that diluted by a tritium monitor 38.

When the tritium gas is much, the gas is accumulated in a used-argontank(s) 28, 29 and/or 37, once, and the partial pressure thereof ismeasured, accurately, again, by the quadrupole mass spectrometer Q-mass,and after being choked, a discharge gas is diluted with the air to beequal to or lower than the reference value depending on that partialpressure, and further the tritium gas in the diluted gas is dischargedwhile monitoring concentration thereof by the tritium monitor 38.

The lithium tank 11 and the lithium pump 17 and the gas systems thereof,into which the tritium therein can be easily collected, are enclosedwithin the hermetically sealed container 7, and also, the gas leaking byany chance, as well as, the gas sucked from the gas system foradjustment of gas pressure are already the tritium therein through thehydrogen isotope removal filter; therefore, it never be diffused intothe atmosphere, directly.

In the flow passage 9 on the discharge side, reaching from the neutronsource 1 to the lithium tank 11 is provided a quench surface fordefining a boundary between a gaseous phase on the neutron source 1 sideand a liquid phase on the lithium tank 11 side, and a position of thesame quench surface 8 is maintained by a very little pressure within thelithium tank 11. With doing this, it is possible, not only to preventthe lithium, flowing from the neutron source 1 into the lithium tank 11,from bringing bubbles into the lithium tank 11, as much as possible, butalso to maintain the position of the quench surface 8 within the flowpassage 9 to be high a little bit due to the very little pressure withinthe lithium tank 11, as well, and bringing NPSH at an inlet of the pumpto be high by combining the head of the lithium liquid and the pressurewithin the lithium tank 11, thereby enabling the pump to operate withstability, without generating cavitations even under a reduced pressure.

The flow passages 9 and 9′ of the lithium are provided level gauges 10and 24, at portions where they penetrate through a wall portion of thehermetically sealed container 7, so as to monitor existence of thelithium in those flow passages 9 and 9′ of the lithium. In particular,at a portion of the quench surface 8 mentioned above, which is providedin the flow passage 9 on discharge side, is provided the level gauge 10for monitoring height of that quench surface 8; thereby achieving astable management.

EFFECT(S) OF THE INVENTION

With the present invention explained in the above, the tritium removaldevice removes, in particular, when the tritium leaks from the lithiumloop or an argon gas system of a cover gas by any chance, the tritiumgas, i.e., the hydrogen and the hydrogen isotope, generating in thelithium flow upon irradiation of protons, and since the lithium targetportion of the neutron source 1 is managed within vacuum, it is possibleto suppress generation of the gas cavitations due to the gas even underthe condition where NPSH is low in the inlet portion of the pump, andthe quench surface is provided within a flow passage 9, further so as toretard a flow velocity of the lithium lower within the lithium tank 11,so that the gas can float up to the surface to be taken out from theargon gas in an upper portion of a lithium tank 11, in other words, itis possible to discharge that gas, after removing the tritium includedin the gas when the tritium leaks out from the lithium tank 11 and thelithium pump 17, etc., i.e., places where the tritium gas is rich, byany chance, and therefore the tritium can be supplied, circularly, withsafe and stability, i.e., preventing the tritium, included in thelithium discharged from the neutron source 1, from being leaked into anoutside.

BRIEF DESCRIPTION OF VIEW OF THE DRAWING

FIG. 1 is a system view of a lithium loop equipped with a tritiumremoval device.

EMBODIMENT(S) FOR CARRYING OUT THE INVENTION

In accordance with the present invention, portion including a lithiumtank 11, for storing lithium discharged from a neutron source, oncetherein, and also a pump, for returning the lithium from this lithiumtank 11 back to the neutron source 1, again, are confined within anair-tight enclosed space of an inactive gas, such as, argon, etc., forexample, wherein tritium accumulating in a gas system of a lithium loopis removed by a tritium removal filter, and further, even when thetritium leaks out, by any chance, from periphery of the lithium tankand/or the pump where hydrogen isotopes including the tritium can easilycome together, by any chance, the tritium to be discharged from aninside of this hermetically sealed container is removed by the tritiumremoval filter, and thereby accomplishing the object mentioned above.

Hereinafter, detailed explanation will be made about the best mode forcarrying out the present invention, by referring to an embodiment.

As is shown in FIG. 1, the neutron source 1 for generating neutrons issectioned into an accelerator 2 for accelerating proton and a neutrongenerating chamber 5 for generating neutrons therein, through a gatevalve 4. The former, i.e., the accelerator 2 is decompressed down tovacuum of 1×10⁻⁵ Pa, approximately, although essentially it is desirableto bring also the neutron generating chamber 5 into a high-vacuumcondition, so that no proton accelerated collides on the gas; however,since the high-vacuum accelerates also evaporation of the lithium, it isnecessary to maintain the vacuum at about vapor pressure of the lithium,such as, 1×10⁻³ Pa, approximately, unwillingly, and therefore it isnecessary to create pressure difference of two digits, compulsively, ina short distance between the accelerator 2 and the neutron generatingchamber 5. Thus, they are connected through a proton passage tube 3,being provided with an operating discharge orifice for building up thepressure difference from the gate valve 4 to the neutron generatingchamber 5. This proton passage tube 3 is decompressed in the pressurethereof, by a lithium vapor trap 27, being filled up with wire nets forcapturing the lithium vapor therein, a turbo molecule pump 26 and a drypump DRP.

To a discharge side of this dry pump DRP are connected the turbomolecule pump TMP and the dry pump DRP, via a hydrogen isotope filtermade of a hydrogen storage alloy, such as, Mg—Ni, etc., a valve, and aquadrupole mass spectrometer Q-mass. An orifice is provided between thevalves in front of the quadrupole mass spectrometer Q-mass, so that theydischarge differentially, thereby obtaining the pressure, under whichthe quadrupole mass spectrometer Q-mass can operate. By means of thisquadrupole mass spectrometer Q-mass is made confirmation on a conditionof removal of tritium after the hydrogen isotope filter, and the gassucked by the turbo molecule pump 26 and the dry pump DRP through thelithium vapor trap 27, after being removed therefrom thehydrogen/hydrogen isotopes through a hydrogen isotope filter 30 made ofthe hydrogen storage alloy, such as, Mg—Ni, etc., is measured onhydrogen partial pressure by the quadrupole mass spectrometer Q-mass,and is discharged by the dry pump DRP into a used-argon tank 37 or 28,while other part of the gas is discharged through the orifice 33 andother remaining gases into the used-argon tank 37. When the used-argontank 28 is filled up with, then the gas is discharged into anotherused-argon tank 29 by the dry pump DRP.

Also, in this gas flow passage system is provided a bypass passage, butthis flow passage system is a route to be used when air comes into thesystem due to the maintenances of a lithium loop and/or a vacuum pump,etc, wherein suction is made by the turbomolecule pump 26 and the drypump DRP from the proton passage tube 3 mentioned above through thelithium vapor trap 27. In this flow passage system, the dry pump DRP isconnected through a hydrogen/oxygen storage alloy built-in filter 31,receiving a hydrogen/oxygen storage alloy, such as, Mg—Ni/Mg, etc.,therein, in the place of such filter receiving the hydrogen storagealloy therein, as was mentioned above. When the air is mixed up with thegas, which is sacked by the turbo molecule pump 26 and the dry pump DRPfrom the proton passage tube 3 through the lithium vapor trap 27, thisgas is sucked by the dry pump DRP through the hydrogen/oxygen storagealloy built-in filter 31. Further, after removing the hydrogen, theoxygen and nitrogen therefrom, passing through a hydrogen-cum-nitrogentrap 32, which receives a vanadium alloy, etc., therein, and is attachedwith a heater on an outside thereof, the remaining gas is dischargedinto the used-argon tank 37. This flow passage system has such structurethat it operates to discharge the gas after passing through the nitrogentrap 32 through an orifice 34, and there is also prepared a bypasspassage for sending the gas to the side of the quadrupole massspectrometer Q-mass mentioned above. Both switching of those passagesare conducted through a valve operation.

In the neutron generating chamber 5 is provided a rectifier plate 6 forforming a thin lithium target flow, and to this rectifier plate 6 isalways supplied the lithium, continuously; therefore a lithium targetflow of a thin laminar flow of lithium is formed on the surface thereof.By letting the protons accelerated by the accelerator 2 to collide uponthe thin lithium target flow on the surface of this rectifier plate 6,neutrons are generated into a direction of collision. In the boronneutron capture therapy (BNCT), a cancer treatment is made by using theneutrons having such direction. Almost all of the protons take electronsin the periphery thereof in this neutron generating chamber 5; therebybecoming hydrogen, and further the tritium, i.e., the hydrogen isotopeis generated from ⁶Li.

After the collision of protons on the lithium target flow on therectifier plate 6 mentioned above, the lithium forming the lithiumtarget flow is sent to the lithium tank 11 passing through a lithiumflow passage 9 provided on a discharge side. An amount of the lithium inthe lithium tank 11 is monitored by a level gauge 13, and within thesame lithium tank is always stocked an approximately certain amount ofthe lithium.

To a bottom portion of the lithium tank 11 is connected a drain tank 12through valves and a dipping-type electromagnetic pump 14. The lithiumwithin this drain tank 12 is monitored by a level gauge 16. When thelithium in the drain tank 12 is in surplus, it is discharged into thedrain tank 12 through an operation of the valve mentioned above, andalso, when the lithium in the lithium tank 11 is in shortage formaintaining a desired liquid level, it is pumped up to the lithium tank11 through driving of the dipping-type electromagnetic pump 14 mentionedabove.

With the lithium tank 11 and the drain tank 12 mentioned above areconnected an argon gas supply system and the dry pump DRP, throughlithium vapor traps 27′ and 27″, each being made up by filling metalnets in a container and for removing the lithium vapor, wherein thoselithium tank 11 and drain tank 12 are maintained under very littleabsolute pressure, such as, about 1 kPa, for example. This pressure of 1kPa corresponds to pressure of a liquid head (i.e., height) of about 200mm of lithium. This pressure is monitored by a pressure gauge P.

When driving, a gas system including the lithium tank 11 and the draintank 12 is brought into a gas communicating condition, to be same in thepressure, and when an emergency occurs, the valve between the lithiumtank 11 and the drain tank 12 is opened, so that the lithium within thelithium tank can be drained, emergently, through a free fall. The reasonthat the lithium tank 11 and the drain tank 12 to be vacuumed, otherthan the neutron generating chamber 5, are brought into the absolutepressure, such as, about 1 kPa, unwillingly, lies in that the lithiumand/or the argon gas will not be discharged into the atmosphere,directly, even if it/they leak(s), since the pressure thereof is lowerthan that of the atmosphere, and further for the purpose of suppressingdischarges of the hydrogen from the lithium and also the tritium gasunder the pressure of argon gas within the lithium loop, down to thesmallest, to enclose the tritium gas within the lithium, as much aspossible therein, when executing the draining emergently, and thereby toincrease the safety when executing the draining emergently. Whenexecuting the draining emergently, also the accelerator is stopped at amoment, automatically, and the gate valve 4 of the neutron generatingchamber 5 in the lithium loop is also stopped at a moment, thereby toseparate the lithium loop, having a very little possibility of producingthe radioactive material therein; however, since the gate valve 4 islarge and it takes time by a unit of 1 second to be completely closed,and since it is necessary to let the argon gas to permeate through theneutron generating chamber 5, firstly, after the lithium is lowered downa hot water surface thereof by a certain degree during the time when theemergency draining of lithium is executed, for the purpose ofmaintaining the vacuum of the accelerator and thereby acceleratingre-starting of the accelerator, then the lithium flow passage 9 isconnected to a lower side portion of the lithium tank 11, so that theargon gas of 1 kPa can be supplied to the neutron generating chamber 5firstly when the lithium goes down to this connecting portion, andthereby allowing the gate valve 4 to take the time unit of 1 second formaking the closure thereof. Although not shown in the figure, stabbingthe lithium flow passage 9 into the lithium tank 11 from an upper flangesurface thereof, i.e., so-called a dual-tube type, also brings about thesimilar effect to that; however, the dual tube makes a diameter of thelithium tank thick, and a liquid-level gauge must be inserted into aring-like portion of the dual tube for the purpose of measuring thelithium surface in the lithium tank 11, but because the lithium haslarge surface tension, the liquid surface of the lithium is absorbedupwards in a narrow region, and this increases an error; therefore, thering-like portion, into which the liquid-level gauge should be inserted,must be widen, considerably. This further brings the diameter of thelithium tank 11 to be thick, as well as, bringing about an increase ofthe amount or volume of the lithium, being a hazardous material; i.e.,not being so preferable method; and therefore, herein, as is shown inFIG. 1, it is dare to select the structure of dividing the flow passage9 the lithium tank 11 from each other, and explain will be given on itwith priority.

In the example shown in the figure, dry pumps DRP are connected on twostages, in series, and to each dry pump DRP on a back thereof areconnected the first used-argon tank 28 and the second used-argon tank29, wherein the hydrogen isotope gas discharged from the gas system,including the lithium tank 11 and the drain tank 12, is sent to ahydrogen isotope filter 42 by the dry pump DRP, and after being removedthe hydrogen isotope gas therefrom, it is accumulated within the argontank 28, and further if the used-argon gas tank 20 is filled up, then itis accumulated in the used-argon gas tank 29 through discharge by thedry pump DRP.

Also the gas accumulated in the used-argon tank 29, finally, is measuredof the hydrogen partial pressure thereof, by the quadrupole massspectrometer Q-mass, being differentially discharged from the used-argontank 37 through the orifice 39, and the gas of the used-argon tank 37 isdiluted with air by means of a blower, so that it comes to be equal toor less than a reference value depending on the tritium partial pressurethereof, and further it is discharged while being monitored the dilutedconcentration thereof by a tritium monitor 38.

Through adjustment of the argon gas pressure by the argon gas supplysystem and the dry pump DRP mentioned above, the liquid level of thelithium within the lithium tank 11 is maintained at a desired height.The lithium flow passage 9 reaching from the neutron generating chamber5 to the lithium tank 11 is connected to the lithium tank 11 at theposition in the middle thereof, being lower than the liquid level of thelithium in the lithium tank 11, so that a liquid surface is maintained,dividing a gaseous phase in an upper side and a liquid phase in a lowerside on the way of the lithium flow passage, as well as, the quenchsurface 8. The fact that this quench surface 8 is positioned on a sideof the lithium flow passage 9 other than the lithium tank 11 and alsothe height thereof bring about a function of preventing bubbles of thehydrogen gas including the tritium therein, from being brought into thelithium tank 11, by the lithium flowing from the neutron source 1 intothe lithium tank 11.

To the lithium tank 11 is also connected the lithium pump 17, beingbuilt up with an induction electromagnetic pump. The lithium in thelithium tank 11 is circulated and supplied to a side of the rectifierplate 6 of the neutron generating chamber 5 mentioned above, passingthrough the lithium flow passage 9′ on the supply side, with driving bythis lithium pump 17. In the lithium flow passage 9′ on the supply sideis provided an electromagnetic flow meter 22, so that a flow amount ofthe lithium, passing through the lithium flow passage 9′ on the supplyside, is measured.

Also, heat exchangers 18 and 23 are provided on the lithium pump 17 andthe lithium flow passage 9′ on the supply side mentioned above,respectively, wherein the heat exchanger 18 cools or heats the lithiumpump 17, while heat exchanger 23 cools or heats the lithium flow passage9′ on the supply side, and thereby achieving an adjustment at thedesired temperature. This lithium, which is adjusted on the temperaturethereof, is supplied to the side of the rectifier plate 6 of the neutrongenerating chamber 5 mentioned above, through the valves. The heatexchangers 18 and 23 for the lithium pump 17 and the lithium flowpassage 9′ on the supply side are cooled or heated, respectively, bycooler/heaters 20 and 21, each applying a heating medium therein, beingheat-resistant against 350° C., thereby being maintained at the desiredtemperatures.

The lithium tank 11, the drain tank 12, the lithium pump 17 and theelectromagnetic flow meter 22 mentioned above are enclosed within ahermetically sealed container 7, including the exchangers 18 and 23 forthe lithium pump 17 and the lithium flow passage 9′ therein. To thishermetically sealed container 7 is connected the dry pump DRP, so thatit is maintained to be negative pressure with respect to the atmosphere.Also, with this hermetically sealed container 7 is connected a argonsupply tank 40 through the valves, and with the argon gas, being sentout from this argon supply tank to the hermetically sealed container 7,an inside of the hermetically sealed container 7 is maintained at aninactive gas atmosphere.

The gas discharged from the same hermetically sealed container 7 by thedry pump DRP, for decompressing the hermetically sealed container 7, isremoved from the hydrogen, the oxygen and the nitrogen thereof, passingthrough the filter 35 made of the hydrogen/oxygen storage alloy, suchas, Mg—Ni/Mg, etc., and also the nitrogen trap 36 made of the vanadiumalloy, as the hydrogen isotope removal filter, and thereafter, theremaining gas is discharged into the used-argon tank 37. This system isattached for the purpose of processing the tritium, if it leaks out fromthe lithium and the argon gas system into the hermetically sealedcontainer 7, by any chance. In case when discharging the gas from theused-argon tank 37, the hydrogen partial pressure is measured by thequadrupole mass spectrometer Q-mass, and the gas in the used-argon tank37 is diluted with the air by the blower, depending on the tritiumpartial pressure thereof, so that it comes to be equal to or less than areference value, and further the gas is discharged while monitoring theconcentration thereof diluted.

The gas discharged into the inside of this used-argon tank 37 ismeasured of the tritium partial pressure, by the quadrupole massspectrometer Q-mass, the gas therein being differentially dischargedthrough the orifice 39, and if the tritium partial pressure is high, thegas is turned from the used-argon discharge tank 37, through a returnpipe, back to the dry pump DRP, and also it circulates around thehydrogen isotope filter 35 and the nitrogen trap 36; thereby after beingremoved the tritium therefrom, it is turned back to the used-argondischarge tank 37. Thereafter, it is discharged, operationally, throughthe orifice 39, and as a result of measurement by the quadrupole massspectrometer Q-mass, if the tritium partial pressure of the gasdischarged into the used-argon discharge tank 37 is low, then the valveis opened to send the gas into a duct 41, wherein the gas is dilutedwith a large amount of air sent by the blower, into the duct 41, to beequal to or lower than a reference in the concentration thereof, and isdischarged into the atmosphere. The tritium concentration of thisdischarge gas is monitored by the tritium monitor.

Within the tritium removal device for the lithium loop having such thestructure as mentioned above, the hydrogen gas including the tritiumtherein, which is accumulated in the gas system for the lithium loop, isremoved by the hydrogen isotope removal filter, and also the gas leakingout from the lithium tank 11 and the lithium pump 17, etc., where thehydrogen gas including the tritium therein can be easily accumulated, byany change, is held within the hermetically sealed container, once, andafter being removed the tritium included in the gas within thishermetically sealed container, said gas is diluted and furtherdischarged into the atmosphere while monitoring it by the tritiummonitor 38. Therefore, a very little amount of the tritium contained inthe lithium discharged from the neutron source 1 is removed by thehydrogen isotope removal filter, and further the partial pressurethereof is measured by the quadrupole mass spectrometer Q-mass, andthereafter it is diluted with the air by means of the blower, to bedischarge while monitoring that the partial pressure thereof is equal toor lower than the reference, by the tritium monitor 38; i.e., it ispossible to supply the lithium, circularly, while preventing the tritiumfrom being leaked directly into an outside.

USABILITY ON INDUSTRY

With the present invention, it can be applied as the tritium removaldevice for removing the tritium, a hydrogen isotope element generatingin the lithium target to be the neutron source, from the lithium loopdistributing the lithium, cyclically, in the in boron neutron capturetherapy (BNCT), which will be applied for the purpose of a cancermedical treatment, for example.

EXPLANATION OF MARKS

1 . . . neutron source

7 . . . hermetically sealed container

8 . . . quench surface

9 . . . flow passage of lithium

9′. . . flow passage of lithium

11 . . . lithium tank

35 . . . filter

1. A tritium removal device for a lithium loop, comprising: a neutronsource (1) for colliding proton rays on a lithium flow, therebygenerating neutrons; a lithium tank (11) for letting the lithium passingthrough this neutron source (1) to flow into through a flow passage (9),forming a quench surface of a lithium flow in the flow passage (9), andfor temporality accumulating it therein, into which an exit of the flowpassage forming the quench surface therein is connected, and therebybringing the lithium tank (11) to be applicable with a gas pressure; anda lithium pump (17) for circulating and supplying the lithium of thislithium tank (11) to said neutron source (1) through a supply-side flowpassage (9′), wherein hydrogen gas containing tritium therein, which isaccumulated in a gas system of the lithium loop is removed by a hydrogenisotope removal filter, and further said lithium tank (11) and saidlithium pump (17) are enclosed within a hermetically sealed container(7), and with this hermetically sealed container (7) is connected adischarge system flow passage through said hydrogen isotope removalfilter.
 2. The tritium removal device for a lithium loop, as describedin the claim 1, wherein a quadrupole mass spectrometer (Q-mass) formeasuring the tritium in said gas of the gas system, from which atritium gas is removed, is attached to a connecting tube, after beingdischarge differentially, and a partial pressure thereof is measured bysaid quadrupole mass spectrometer (Q-mass), and thereby dilution is madewith air to be equal to or lower than a reference value depending onthat partial pressure.
 3. The tritium removal device for a lithium loop,as described in the claim 1, wherein the tritium gas is accumulated in aused-argon tank(s) (28), (29) and/or (37), once, so as to measure thepartial pressure thereof, again, by the quadrupole mass spectrometer(Q-mass), and after being choked, a discharge gas is diluted with theair to be equal to or lower than the reference value depending on thatpartial pressure.
 4. The tritium removal device for a lithium loop, asdescribed in the claim 2, wherein the tritium gas in the diluted gas isdischarged while monitoring concentration thereof by a tritium monitor(38).
 5. The tritium removal device for a lithium loop, as described inclaim 1, wherein the quench surface (8) for defining a boundary betweena gaseous phase of said neutron source (1) side and a liquid phase ofsaid lithium tank (11) side is provided in said flow passage (9) ofdischarge side, reaching from said neutron source (1) to said lithiumtank (11).
 6. The tritium removal device for a lithium loop, asdescribed in the claim 5, wherein a position of said quench surface (8)of said flow passage (9) is held by pressure within said lithium tank(11).
 7. The tritium removal device for a lithium loop, as described inclaim 1, wherein level gauges (10) and (24) are provided in portions ofsaid passages (9) and (9′), at which they penetrate through a wallportion of said hermetically sealed container (7), so as to monitorexistence of the lithium in those flow passages (9) and (9′) for thelithium.